PSI - Issue 68

Koji Uenishi et al. / Procedia Structural Integrity 68 (2025) 554–558 Uenishi et al. / Structural Integrity Procedia 00 (2025) 000–000

557

4

a

b

0 µ s

5 µ s

10 mm

Fig. 2. (a) Same as in Fig. 1, but now the specimen has a straight perforation line with relatively coarsely distributed small-scale cracks illustrated in (b) [unit: mm]. Again, quasi-static tension at a constant strain rate of 1.67 ´ 10 - 1 /s is applied to the specimen, and the fracture behavior is observed with a high-speed video camera at a frame rate of 200,000 fps. The fluctuation of fracture propagation speed at supershear levels seems to generate multiple Mach wavefronts with different Mach angles indicated in red. perforation line, but its propagation speed is not constant. The speed rather fluctuates between subsonic and supershear levels, with the maximum just before the fracture leaves the edge of a preexisting small-scale crack and low values just before the fracture approaches the edge of the next crack. It is possible that the (pre-)existence of the small-scale cracks can accelerate the fracture propagation speed to a higher level. If the small-scale cracks along the perforation line is less densely distributed, the speed of fracture propagation may fluctuate at supershear levels, generating multiple Mach wavefronts around the tip of the propagating fracture, but its mechanical details are still open.